The present invention relates to a security document having an embedded security element in the form of a thread or strip comprising a carrier sheet preferably made of plastic material and a metal coating.
It is known to protect security documents, in particular securities, bank notes, identity cards or the like, from unauthorized imitation by embedding so-called "security threads". The embedding in the paper mass of a bank note constitutes a great obstacle for forgers since such embedding can only be performed during the production of the paper and not subsequently. In addition, security threads are used which are characterized by special properties and can be tested for authenticity in the embedded state by machine and/or visually. A frequent test criterion is e.g. the electrical conductivity.
It is known from Swiss patent no. 472 081, for example, to provide bank notes with metal threads which can be tested for electrical conductivity or their response to X-rays. However, these metal threads are relatively inflexible and break easily during daily use. The electrical conductivity can thus be measured only in some areas and no longer over the entire width of the bank note.
One has therefore begun providing rayon threads or plastic sheets with a metal coating and embedding these sheets in bank note paper in the form of strips (German patents nos. 640 232, 27 54 267). This thread is characterized by high flexibility but in practice the metal coating very often shows cross-cracks which lead to an interruption in the electrical conductivity, so that this feature can again not be used as an unambiguous authenticity feature or involves high reject rates during later testing.
The reason for these cross-cracks is presumably that the security thread is subjected to high tensile stress when it is embedded in the paper, so that the thread is stretched and this interrupts the metal layer which is rigid compared to the plastic material.
The invention is therefore based on the problem of providing a security document having a security thread embedded therein that possesses a predetermined electrical conductivity contributing to the authenticity of the paper, this conductivity being maintained over the entire length of the thread even under tensile stresses that occur during embedding in security papers.
This problem is solved by the features contained in the characterizing part of the main claim. Advantageous developments are the subject of the subclaims.